1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to a method and apparatus of measuring and adjusting a response speed of a liquid crystal display device for automatically establishing an adjusted driver voltage according to modulating data.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) device controls light transmittance of individual liquid crystal cells according to a video signal, thereby displaying an image. An active matrix LCD device includes a switching device for the individual liquid crystal cells, thereby displaying dynamic images (i.e., moving pictures). Thin film transistors (TFT""s) are commonly used in the active matrix LCD as the switching devices. However, LCD devices are disadvantageous since they have relatively slow response characteristics due to inherent electro-mechanical properties of the liquid crystal material, such as viscosity and elasticity. Equation (1) demonstrates the dependency of the response characteristics upon the electro-mechanical properties of the liquid crystal material during a period of increasing applied voltage:
xcfx84rxe2x88x9dxcex3d2/xcex94xcex5|V2axe2x88x92V2F|xe2x80x83xe2x80x83(1)
wherein xcfx84r is a rise time when a voltage is applied to liquid crystal molecules of the liquid crystal material, Va is the voltage applied to the liquid crystal molecules, VF is a Freederick transition voltage when the liquid crystal molecules begin an inclined motion, xe2x80x9cdxe2x80x9d is a cell gap of the individual liquid crystal cells, and xcex3 is a rotational viscosity of the liquid crystal molecules.
In addition, Equation (2) demonstrates the dependency of the response characteristics upon the electro-mechanical properties of the liquid crystal material during a period of decreasing applied voltage:
xcfx84fxe2x88x9dxcex3d2/Kxe2x80x83xe2x80x83(2)
wherein xcfx84f represents a fall time when the liquid crystal molecules return to an initial position by an elastic restoring force after the voltage applied to the liquid crystal molescules is removed, and K is an elastic constant of the liquid crystal molecules.
FIG. 1 is a schematic diagram of a response speed measuring device employing an oscilloscope according to the related art. In FIG. 1, a response speed measuring device employing an oscilloscope including a pattern generator 11 for applying a two-level pulse to a liquid crystal display panel 12, a photo detector 13 for detecting light intensity of a sample image pattern displayed on the liquid crystal display panel 12, and an oscilloscope 14 connected to the photo detector 13.
The pattern generator 11 generates a specific frequency of two-level pulses and applies it to the liquid crystal display panel 12. The liquid crystal display panel 12 has a liquid crystal material injected between two glass substrates, and data and gate lines are orthogonally positioned on the lower glass substrate. A thin film transistor (TFT) is provided at each intersection between the data and gate lines, wherein the TFT responds to a scanning pulse to supply data to the data lines of a liquid crystal cell. The liquid crystal display panel 12 displays a sample image depending upon a two-level pulse input from the pattern generator 11.
The photo detector 13 performs a photo-electric conversion of light input from a sample image displayed on the liquid crystal display panel 12, wherein a current output from the photo detector 13 is proportional to an intensity of the light. The oscilloscope 14 converts a current signal output from the photo detector 13 into a voltage signal and displays the converted signal on a display screen, thereby detecting a response characteristic of the liquid crystal display panel 12.
FIG. 2 is a response speed measuring device employing an electro-optic characteristic device according to the related art. In FIG. 2, a response speed measuring device employing an electro-optic characteristic device including a pattern generator 21 for applying a two-level pulse to a liquid crystal display panel 22, a photo detector 23 for detecting light intensity of a sample pattern image displayed on the liquid crystal display panel 22, and a photo-multiplier tube (PMT) 24 connected between the photo detector 23 and the pattern generator 21.
The pattern generator 21 generates a specific frequency of two-level pulses and applies it to the liquid crystal display panel 22. The pattern generator 21 includes a monitor for displaying a signal output from the PMT 24 and a driving circuit for the monitor, thereby displaying the signal output from the PMT 24 on the screen of the monitor. Here, the liquid crystal display panel 22 is substantially identical to the liquid crystal display panel 12 shown in FIG. 1.
The photo detector 23 generates a photo-electric conversion of light input from a sample image displayed on the liquid crystal display panel 22, wherein a current output from the photo detector 23 is proportional to an intensity of the light. The PMT 24 converts an analog current signal input from the photo detector 23 into a digital voltage signal suitable for the pattern generator 21, thereby applying the digital voltage signal to the pattern generator 21.
FIG. 3 is a waveform diagram of a response characteristic of a two-level pulse response speed measuring device according to the related art. In FIG. 3, a liquid crystal response speed characteristic device applies a two-level pulse to the liquid crystal display panels (12 and 22 in FIGS. 1 and 2, respectively) to measure a liquid crystal response characteristic (LCRT) of the liquid crystal display panels 12 or 22 according to the pulse signal.
In FIG. 3, the liquid crystal response characteristic (LCRT) is changed from a low-level into a high-level at a rise time xcfx84r defined by Equation (1) and is changed from a high-level into a low-level at a fall time xcfx84f defined by Equation (2). Accordingly, the rise time xcfx84r is measured by an interval ranging from 10% charging time until 90% charging time within a time interval when the LCRT is changed from a low-level into a high-level. The fall time xcfx84f is measured by an interval ranging from 10% discharging time until 90% discharging interval within a time interval when the liquid crystal response characteristic is discharged from a high-level into a low-level.
FIG. 4 is a response characteristic diagram of a dynamic image of a liquid crystal display device according to the related art. A twisted nematic (TN) mode liquid crystal has a different response speed due to a electro-mechanical characteristic of liquid crystal material positioned within the cell gap. In general, the response speed has a rise time of 20 to 80 ms and a fall time of 20 to 30 ms. Accordingly, since the liquid crystal material has a response speed longer than one frame interval of a moving image (i.e., 16.67 ms in the case of an NTSC system), a voltage charged in the liquid crystal cell progresses into the next frame prior to arriving at a desired characteristic, as shown in FIG. 4. Thus, a blurring phenomenon causes blurring successive images on the display panel.
In FIG. 4, the LCD device cannot generate a desired color and brightness since, upon implementation of a moving image, a display brightness BL fails to achieve a target brightness that corresponds to a change of a data VD from one level into another level due to its slow response speed. Accordingly, in the LCD device, the blurring phenomenon appears from the moving images, and display quality deteriorates due to a reduced contrast ratio. To overcome this, modulation of source data is performed using a pre-determined modulating data in accordance with look-up tables (i.e., high-speed driving strategy), as demonstrated by U.S. Pat. No. 5,495,265 or PCT International Publication No. WO99/05567, which are hereby incorporated by reference.
The high-speed driving strategy increases the quantity |Va2xe2x88x92VF2| from Equation (1) upon a basis of status change of the data so that a desired brightness can be obtained in response to a brightness value of an input data within one frame interval, thereby accelerating a response speed of the liquid crystal material. Accordingly, the LCD employing such a high-speed driving strategy compensates for a slow response speed of the liquid crystal material by means of modulating a data value to alleviate the blurring phenomenon from moving images. Thus, an image at a desired color and brightness is displayed.
In the high-speed driving strategy, the modulating data changes while changing a voltage and a pulse width of the two-level pulse until a response speed of the liquid crystal achieves a desired level by the response speed measuring device, as shown in FIGS. 1 and 2. For this reason, adjustment of the two-level pulse and measurement of the response speed are carried out repetitively to establish the modulating data using the high-speed driving strategy. As a result, the conventional response speed measuring device according to the related art that uses the modulating data method is problematic because it requires a significant amount of time and has poor accuracy.
Accordingly, the present invention is directed to a method and apparatus for measuring and adjusting a response time of liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method and apparatus of measuring a response speed of a liquid crystal display device that automatically establishes a modulating data for determining driving voltage.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, a method of measuring a liquid crystal response speed includes steps of generating a sample pulse having a target voltage level and a variable voltage level that varies according to a response characteristic of a display panel, applying the sample pulse to the display panel, detecting the response characteristic of the display panel by the sample pulse, adjusting the variable voltage level until a desired level of the response characteristic is obtained, and setting the variable voltage level according to modulating data when the desired level of the response characteristic is obtained.
In another aspect, a system for measuring liquid crystal response speed includes a generator system for generating a sample pulse having a first voltage level and a variable second voltage level, a display system for receiving the sample pulse to display an image, a response detector system for detecting a varying response characteristic of the display system by the sample pulse, and a controller system for adjusting and setting the variable second voltage level.
In another aspect, a system for measuring liquid crystal response speed includes a generator system for generating a sample pulse having a first voltage level and a variable second voltage level, a display system for receiving the sample pulse to display an image, a response detector system for detecting a varying response characteristic of the display system by the sample pulse, and a controller system for adjusting and setting the variable second voltage level.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.